A femtosecond laser may continuously generate a relatively short optical pulse of a femtosecond scale on a predetermined cycle. Femtosecond laser technology has a relatively short history of commercialization and industrial field utilization. Research on the femtosecond laser and various application fields related to the femtosecond laser is actively being conducted by many research institutions around the world.
Generally, a reference signal source that generates a reference signal is required to measure a phase noise of a repetition rate of an optical pulse train generated by a femtosecond pulse laser. That is, a noise is measured based on a method of comparing the reference signal to a signal to be measured. Thus, the reference signal may have noise of which a level is lower than or similar to that of the signal to be measured. Therefore, when the signal to be measured is an ultra low noise signal, there may be a difficulty in that two identical signal sources are required.
A method of measuring a phase noise of a radio frequency (RF) signal or a microwave signal may include a method of measuring the phase noise using one signal source. The method may guide the signal to be measured to two paths and then delay a signal of any one of the paths using an optical fiber link to measure a noise by comparing the delayed signal to a signal of the other path. However, there is a limit to measurement performance in case of applying the method to a method of measuring a phase noise of a repetition rate of a pulse laser. The related methods of measuring the phase noise of the repetition rate of the pulse laser without an additional signal source may not have a sufficiently high resolution power such that the methods are unable to measure the phase noise throughout a wide Fourier frequency.
Thus, there is a need for a technology capable of measuring the phase noise of the repetition rate of the pulse laser having a high resolution power without an additional signal source.